The present state-of-the-art in semiconductor radiation detection is silicon diodes, high purity germanium (cooled by liquid nitrogen), and compound semiconductors, such as CZT and mercuric iodide. Each of these materials has one or more significant drawbacks related to its use. Silicon has a low atomic number and is therefore primarily useful for the detection of energetic charged particles and atomic x-rays emitted by low atomic number elements. Germanium has a higher atomic number but, because of its low band gap energy, must be cooled by liquid nitrogen in a bulky, expensive, and potentially dangerous cryogenic system in order to reduce thermally generated noise. Compound semiconductors, such as CZT and mercuric iodide, have sufficiently high band gap energy to be useful at or near room temperature. However, CZT has been plagued by production problems, resulting in polycrystalline ingots with twins, inclusions, and grain boundary defects. These defects can never be completely removed and are a consequence of CZT being a solid solution, rather than a true compound. The result is that spectroscopy grade crystals must be mined from bulk material. Mercuric iodide suffers from low carrier mobility, short carrier lifetime, space charge polarization, and surface degradation. In addition, mercuric iodide is an extremely soft material that is easily damaged by the slight pressure of an electrical connection and by temperatures over sixty degrees Celsius. In general, these compound semiconductors do not interact well with neutrons such that they must be coupled with a thin layer of a neutron absorbing material, such as 6LiF or 10B. A reaction between 6Li or 10B occurs in the thin absorber layer, which creates alpha particles that are detected by a semiconducting substrate. The absorber layer must be thin in order for the semiconducting substrate to detect the resultant alpha particles. 3He gas filled tube detectors are the state-of-the-art for thermal neutron detection.
As a result, U.S. Pat. No. 7,687,780 (Bell et al.) provides a semiconductor detector of ionizing electromagnetic radiation, neutrons, and energetic charged particles. The detecting element includes a compound having the composition I-III-VI2 or II-IV-V2, where the “I” component is from column 1A or 1B of the periodic table, the “II” component is from column 2B of the periodic table, the “III” component is from column 3A of the periodic table, the “IV” component is from column 4A of the periodic table, the “V” component is from column 5A of the periodic table, and the “VI” component is from column 6A of the periodic table. The detecting element detects ionizing electromagnetic radiation by generating a signal proportional to the energy deposited in the element, and detects neutrons by virtue of the ionizing electromagnetic radiation emitted by one or more of the constituent materials subsequent to capture. The detector may contain more than one neutron sensitive component.
Related to the I-III-VI2 compounds, however, improved methods for combining the elemental constituents in a multistep synthetic process are still required, providing improved purity and homogeneity and more precisely controlling the reaction rate and yielding a I-III-VI2 charge with a single phase stoichiometry.
Further, a worldwide helium shortage has developed in recent years due to the limited supply of 3He and an increasing demand for neutron detection materials for scientific and security applications. As a result, research into alternatives to gas detectors (3He or 10BF3) or scintillation detectors (cerium-doped lithium glass) has grown. The latter of these are generally called elpasolites and one, CLYC, has been shown to be a dual neutron/gamma detector. However, in practice, CLYC has serious manufacturing, cost, and stability problems. It is highly desirable to develop an improved solid-state detector for neutron detection. In much the same way as solid-state CZT detectors have revolutionized gamma-ray detection, a solid-state neutron detector would offer the significant advantages of portability, sensitivity, simplicity, and low cost. A neutron absorber (e.g., 6Li or 10B) must be used along with a charge collector in such a device. To date, most reports of lithium containing solid-state neutron detectors have used a lithium conversion layer in conjunction with a silicon diode detector. To obtain a useable thickness of lithium to stop neutrons efficiently, deep holes are etched into the silicon, and a lithium (or boron) containing material is conformally deposited into the holes. If, however, the neutron absorber is within the charge generating/collecting device, which is the semiconductor that each thermal neutron impinging on the detector crystal has a high probability of reacting with, the absorber atom inside the solid generates high-energy charged particles that, in turn, use their energy to create electron-hole pairs in the semiconductor. This is the same phenomenon that occurs in coated semiconductor detectors; however, in the Li containing compound semiconductor, the charged particles are created within the charge collection device, and the entire Q-value of the reaction is available for charge generation. In the coated detector, however, only one of the two charged particles can enter the detector, and that alpha particle must have lost a random fraction of its energy traversing the absorber layer. Ternary I-III-VI2 semiconductors with a chalcopyrite crystal structure type would meet these criteria if 6Li is included, for example.